User Experience is about happiness and well-being. As I wrote elsewhere: "Usability wants us to die rich, UX wants us to die happy." In that sense, Experience Design is about designing interventions, which make people feel better - or happier? - or make their lives more meaningful? - or less miserable? You get it ... this is where the trouble starts. If we want to design for experience, we not only need to think about what experience is, but also what our design goal is. Because, as I would argue, there are plenty of different joys and forms of happiness, each following its own set of rules. When looking for theory and research on happiness, Positive Psychology is a first place to stop. Initiated by the prominent psychologists Martin Seligman and Mihaly Csikszentmihaly Positive Psychology devotes itself to the study of the positive sides of human life. Far from settled and from my point of view, sometimes with the touch of esoteric self-help, Positive Psychology however asks the right questions.
Have a look at Martin Seligman's engaging introduction to Positive Psychology.